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Answers to In-Chapter Critical Thinking Questions
Chapter 1
A Brief History of Microbiology
p. 5
A few bacteria produce disease because they derive nutrition from human cells and
produce toxic wastes. Algae do not cause disease. Why not?
Algae have simple nutritional requirements (carbon dioxide, water, light, a few salts)
that are easily obtained from the environment; therefore, they do not need to obtain
nutrients from other living things.
p. 11
How might the debate over spontaneous generation have been different if Buchner
had conducted his experiments in 1857 instead of 1897?
Buchner’s experiments demonstrated that intact living cells were not required for
fermentation of sugars. Had the experiments been done in the 1850s, prior to
Pasteur’s experiments on spontaneous generation, one argument for spontaneous
generation (the appearance of yeast cells in fermenting sugar solutions) would have
been weakened. Spallanzani’s results might have been more widely accepted as
demonstrating that spontaneous generation does not occur. When scientists accepted
that animals do not arise from spontaneous generation, they might have been less
likely to exempt microorganisms.
p. 14
French microbiologists, led by Pasteur, tried to isolate a single bacterium by diluting liquid media until only a single type of bacterium could be microscopically
observed in a sample of the diluted medium. What advantages does Koch’s method
have over the French method?
Many bacteria are very small and may be easily overlooked during examination with
a light microscope. In contrast, Koch’s method allows a single microbe to reproduce
into a much more visible population of cells—a colony—which is less likely to be
overlooked. In addition, many microorganisms, especially pathogens, have stringent
nutritional requirements; dilution in standard growth medium might be harmful to
the microbes and therefore prevent their detection.
p. 14
Why aren’t Koch’s postulates always useful in proving the cause of a given disease?
Consider a variety of diseases, such as cholera, pneumonia, Alzheimer’s, AIDS,
Down syndrome, and lung cancer.
Koch’s postulates are not useful in determining the cause of diseases that are not
infections (e.g., Down syndrome, lung cancer). Some diseases (e.g., cholera) occur
only under specific conditions (high numbers, specific genetic component), so the
microbes may be present in asymptomatic persons, and Koch’s first postulate is not
met. Disease caused by microorganisms that are infectious only in humans is
difficult to prove by Koch’s postulates because ethical considerations prohibit
deliberate exposure of humans to potential harm (e.g., HIV/AIDS), preventing the
third postulate from being applied. Some syndromes are common to a variety of
microbes (e.g., pneumonia), so identifying a single causative agent is not possible
(first postulate again). Alzheimer’s disease may be a physiologic/genetic disorder
and not suitable for the application of Koch’s postulates, or it may be a syndrome
caused by a nonliving agent (prion), which also precludes application of the
postulates.
p. 19
Albert Kluyver said, “From elephant to . . . bacterium—it is all the same!” What did
he mean?
Kluyver was referring to the metabolism/biochemistry that is common to all cellular
life on the planet, regardless of the number of cells per organism or size. The
statement can be extended to shared cellular structures as well.
p. 20
The ability of farmers around the world to produce crops such as corn, wheat, and
rice is often limited by the lack of nitrogen-based fertilizer. How might scientists use
Beijerinck’s discovery to increase world supplies of grain?
Nitrogen compounds (NO3, NO2, NH3, etc.) are often a limited resource for growing
plants, but nitrogen gas (N2) is abundant in the air. Beijerinck’s discovery that some
bacteria can convert nitrogen gas to nitrogen compounds is important to the
cultivation of these grains. It may be possible to (1) introduce into the soil bacteria
capable of converting N2 to organic forms of nitrogen, (2) create soil conditions that
promote the growth of such bacteria, and/or (3) genetically modify crop plants with
bacterial genes to convert nitrogen gas for themselves.
Chapter 2
The Chemistry of Microbiology
p. 29
Neon (atomic mass 10) and argon (atomic mass 18) are inert elements, which means
that they very rarely form chemical bonds. Give the electron configurations of their
atoms and explain why these elements are inert.
Neon has two electrons in the inner shell and eight electrons in the outer cloud/shell.
Argon has two in the innermost cloud/shell, eight electrons in the middle cloud, and
eight electrons in the outer cloud/shell. An eight-electron cloud is a highly stable
(low-energy) state and has no more room to add electrons. Losing an electron greatly
decreases the stability, so not sharing electrons with other atoms is a more stable
(lower-energy) state than “giving away” or “accepting” electrons. Because electron
sharing (or loss) is the basis for chemical interactions, these elements do not interact
with other atoms.
p. 31
An article in the local newspaper about gangrene states that the tissue-destroying
toxin, lecithinase, is an “organic compound.” But many people consider “organic”
chemicals to mean something is good. Explain the apparent contradiction.
Chemists use the term organic to mean compounds that contain carbon and
hydrogen, which includes all biomolecules (“life molecules”). Nonscientists use the
term organic to mean “from nature,” which is perceived as good or better than
synthetic. There is therefore some overlap: both groups use the term organic to mean
biomolecules, but the latter position overlooks the fact that many natural processes
produce toxins (e.g., lecithinase, botulism toxin, rattlesnake venom).
p. 32
The deadly poison cyanide has the chemical formula H–CN. Describe the bonds
between carbon and hydrogen, and between carbon and nitrogen, in terms of the
number of electrons involved.
Triple covalent bonds are stronger and more difficult to break than single
covalent bonds. Explain the reason why by referring to the stability of a valence
shell that contains eight electrons.
Hydrogen shares its single electron with carbon. The carbon shares one electron with
the hydrogen and three electrons with nitrogen.
Both atoms in a triple bond have a more stable state when the valence shell is
“full” with eight electrons. Removing three electrons simultaneously (breaking the
triple bond) requires much more energy than removing a single electron.
p. 33
According to the chart in Figure 2.6, what type of bond (nonpolar covalent, polar
covalent, or ionic) would you expect between chlorine and potassium? Between
carbon and nitrogen? Between phosphorous and oxygen? Explain your reasoning in
each case.
The bond between K and Cl is an ionic bond. Chlorine is much more electronegative
than potassium and “steals” an electron from potassium.
The bond between C and N is a nonpolar covalent bond. The electronegativity
of carbon and nitrogen is nearly equal, so electrons are shared essentially equally.
The bond between P and O is a polar covalent bond. Oxygen is somewhat more
electronegative than phosphorous, so an electron is shared between the atoms,
although unequally—the electron spends more time in the vicinity of the O nucleus
than of the P nucleus.
p. 36
How can hydrogen bonding between water molecules help explain water’s ability to
absorb large amounts of energy before evaporating?
The polar nature of the bonds between H and O and the nonlinear structure of the
water molecule mean that each H2O molecule is usually participating in three or four
hydrogen bond pairs. Energy is required to break these bonds, and three to four
bonds need to be broken for each water molecule to escape the liquid and evaporate.
p. 38
How can a single molecule of magnesium hydroxide neutralize two molecules of
hydrochloric acid?
Magnesium hydroxide dissociates to release two OH–, whereas each molecule of
HCl dissociates to release only one H+.
p. 40
We have seen that it is important that biological membranes remain flexible. Most
bacteria lack sterols in their membranes and instead incorporate unsaturated phospholipids in the membranes to resist tight packing. Reexamine Table 2.4. Which
fatty acid might best protect the membranes of an ice-dwelling bacterium?
Linoleic acid is the least linear of the fatty acids in Table 2.4 and will best interfere
with tight packing of the fatty acids in bacterial membranes that would otherwise
occur at cold temperatures.
p. 44
Why isn’t there a stereoisomer of glycine?
Glycine’s R group is hydrogen. Glycine therefore has two hydrogens attached to the
central carbon, and its “mirror images” are identical; that is, there are no
stereoisomers.
p. 47
A textbook states that only five nucleotide bases are found in cells, but a laboratory
worker correctly reports that she has isolated eight different nucleotides. Explain
why both are correct.
Living organisms use five nucleotide bases (adenine, cytosine, guanine, thymine,
and uracil). Cells combine A, C, or T nucleotide bases with either of two different
sugars (deoxyribose and ribose) to form nucleotides (for DNA and RNA,
respectively). Thymine is combined only with deoxyribose, and uracil is combined
only with ribose; therefore, there are eight nucleotides [(3  2) + 1 + 1 = 8].